# Homework Help: Force of magnetic attraction between hemispheres

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1. Nov 9, 2017

### Pouyan

1. The problem statement, all variables and given/known data

Calculate the force of magnetic attraction between the northern and southern hemispheres of a uniformly charged spinning spherical shell, with radius R, angular velocity ω and surface charge density σ.

2. Relevant equations
Maxwell Tensor : Tm =
(1/μ) * ((B*n)B - (1/2)*B2 n)

B_in =
(2/3)*μσRω*z
B_out =
μm/(4πr3) * (2cosθ r + sinθ θ)
where
m = (4/3)*πR3(σωR)

3. The attempt at a solution
I do see in my solution that :
1-We use a surface consisting of the entire equatorial plane, CLOSING IT with a hemispherical surface at infinity where (since the field is zero out there) contribution is zero.
for r>R :
B = μm/(4πr3) θ = - μm/(4πr3) z (Why ?! )
2-
We have a equatorial circular disk. We use B inside and da = rdrdφ. Direction is in z-axis!
I don't want the solution because I already have it but MY QUESTIONS are :

A) How should think and imagine this spheres ?!
Should I think like this and use those formulas :

B) Why we do write B = μm/(4πr3) θ = - μm/(4πr3) z
Why is θ =-z in this case ?! I dont get it !

I know the rest. That is just integrating

2. Nov 9, 2017

### TSny

I'm not quite following what you are asking here. Can you reword the question?

It is because you are using spherical coordinates with θ measured from the positive z axis. The unit vector θ points in the direction of increasing θ. For a point on the equatorial plane of the sphere, what is the direction of θ?

3. Nov 10, 2017

### Pouyan

I've got it !

Thanks